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Updated for Xcode 15
Trailing closure syntax is designed to make Swift code easier to read, although some prefer to avoid it.
Let’s start with a simple example first. Here’s a function that accepts a Double
then a closure full of changes to make:
func animate(duration: Double, animations: () -> Void) {
print("Starting a \(duration) second animation…")
animations()
}
(In case you were wondering, that’s a simplified version of a real and very common UIKit function!)
We can call that function without a trailing closure like this:
animate(duration: 3, animations: {
print("Fade out the image")
})
That’s very common. Many people don’t use trailing closures, and that’s OK. But many more Swift developers look at the })
at the end and wince a little – it isn’t pleasant.
Trailing closures allow us to clean that up, while also removing the animations
parameter label. That same function call becomes this:
animate(duration: 3) {
print("Fade out the image")
}
Trailing closures work best when their meaning is directly attached to the name of the function – you can see what the closure is doing because the function is called animate()
.
If you’re not sure whether to use trailing closures or not, my advice is to start using them everywhere. Once you’ve given them a month or two you’ll have enough usage to look back and decide more clearly, but hopefully you get used to them because they are really common in Swift!
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